ITGB6 x Tubulin

This study compared the effect of the transcription factor Crx (cone, rod homeobox) on the differentiation of human adult corneal (hCSC) and retinal (hRSC) stem cells into functional photoreceptors.

Stem cells isolated from postmortem human corneas and retinal ciliary bodies were maintained in serum-free culture and genetically modified by electroporation to express exogenous epitope-tagged murine Crx. Expression of stem cell markers (Pax6, Oct3/4, and proliferating cell nuclear antigen [PCNA]), neuronal markers (nestin, neuron-specific class III beta-tubulin, Map2 a/b, and neurofilament), and photoreceptor-specific markers (rhodopsin, cyclic nucleotide-gated cation channel-3, blue-cone opsin, and beta-6-PDE) was evaluated by immunocytochemistry. A cGMP enzyme-linked immunoassay was used to assess phototransduction cascade activity by measurement of light-induced hydrolysis of cGMP.

Expression of the stem cell markers of proliferation and pluripotency Pax6, PCNA, and Oct3/4 was decreased by exogenous Crx expression in both hCSCs and hRSCs. Correspondingly, the expression of the mature neuronal markers Map2 a/b and neurofilament was increased. Both hCSCs and hRSCs displayed photoreceptor-specific immunolabeling. However, light-activated GMP hydrolysis was observed only in hRSCs after exogenous expression of Crx.

The present study extends previous findings that exogenous Crx expression can promote differentiation of human retina-derived stem cells into light-sensitive photoreceptor phenotypes. Although Crx can induce human cornea-derived stem cells to express photoreceptor-specific proteins, it does not seem to be sufficient to direct their differentiation into functional photoreceptors. Nevertheless, this study demonstrates that genetic modification of adult human retinal stem cells can cause differentiation into light-sensitive photoreceptor phenotypes.

Nicotinic acetylcholine receptors (nAChRs) mediate fast cholinergic synaptic transmission at nerve-muscle junctions and in the brain. However, the complete gene family of nAChRs has not so far been reported for any vertebrate organism. We have identified the complete nAChR gene family from the reference genome of the pufferfish, Fugu rubripes. It consists of 16 alpha and 12 non-alpha candidate subunits, making it the largest vertebrate nAChR gene family known to date. The gene family includes an unusual set of muscle-like nAChR subunits comprising two alpha1s, two beta1s, one delta, one epsilon, and one gamma. One of the beta1 subunits possesses an aspartate residue and N-glycosylation sites hitherto shown to be necessary for delta-subunit function. Potential Fugu orthologs of neuronal nAChR subunits alpha2-4, alpha6, and beta2-4 have been identified. Interestingly, the Fugu alpha5 counterpart appears to be a non-alpha subunit. Fugu possesses an expanded set of alpha7-9-like subunits and no alpha10 ortholog has been found. Two new candidate beta subtypes, designated beta5 and beta6, may represent subunits yet to be found in the human genome. The Fugu nAChR gene structures are considerably more diverse than those of higher vertebrates, with evidence of "intron gain" in many cases. We show, using RT-PCR, that the Fugu nAChR subunits are expressed in a variety of tissues.

Clostridium botulinum C3 exoenzyme inactivates the small GTP-binding protein family Rho by ADP-ribosylating asparagine 41, which depolymerizes the actin cytoskeleton. C3 thus represents a major family of the bacterial toxins that transfer the ADP-ribose moiety of NAD to specific amino acids in acceptor proteins to modify key biological activities in eukaryotic cells, including protein synthesis, differentiation, transformation, and intracellular signaling. The 1.7 A resolution C3 exoenzyme structure establishes the conserved features of the core NAD-binding beta-sandwich fold with other ADP-ribosylating toxins despite little sequence conservation. Importantly, the central core of the C3 exoenzyme structure is distinguished by the absence of an active site loop observed in many other ADP-ribosylating toxins. Unlike the ADP-ribosylating toxins that possess the active site loop near the central core, the C3 exoenzyme replaces the active site loop with an alpha-helix, alpha3. Moreover, structural and sequence similarities with the catalytic domain of vegetative insecticidal protein 2 (VIP2), an actin ADP-ribosyltransferase, unexpectedly implicates two adjacent, protruding turns, which join beta5 and beta6 of the toxin core fold, as a novel recognition specificity motif for this newly defined toxin family. Turn 1 evidently positions the solvent-exposed, aromatic side-chain of Phe209 to interact with the hydrophobic region of Rho adjacent to its GTP-binding site. Turn 2 evidently both places the Gln212 side-chain for hydrogen bonding to recognize Rho Asn41 for nucleophilic attack on the anomeric carbon of NAD ribose and holds the key Glu214 catalytic side-chain in the adjacent catalytic pocket. This proposed bipartite ADP-ribosylating toxin turn-turn (ARTT) motif places the VIP2 and C3 toxin classes into a single ARTT family characterized by analogous target protein recognition via turn 1 aromatic and turn 2 hydrogen-bonding side-chain moieties. Turn 2 centrally anchors the catalytic Glu214 within the ARTT motif, and furthermore distinguishes the C3 toxin class by a conserved turn 2 Gln and the VIP2 binary toxin class by a conserved turn 2 Glu for appropriate target side-chain hydrogen-bonding recognition. Taken together, these structural results provide a molecular basis for understanding the coupled activity and recognition specificity for C3 and for the newly defined ARTT toxin family, which acts in the depolymerization of the actin cytoskeleton. This beta5 to beta6 region of the toxin fold represents an experimentally testable and potentially general recognition motif region for other ADP-ribosylating toxins that have a similar beta-structure framework.